The invention relates to flat or tubular cellulose-based food casings which are produced by extruding (xe2x80x9cspinningxe2x80x9d) cellulose dissolved in N-methylmorpholine N-oxide. The casings are particularly suitable as sausage casings.
Cellulose is insoluble in the usual solvents. It does not have a melting point or melting range and cannot therefore be melt-processed either. Therefore, it is usually chemically modified for producing food casings. However, these processes are associated with a breakdown of the cellulose, i.e. the mean degree of polymerization of the cellulose becomes lower. In addition, the processes are highly technically complex and correspondingly expensive.
Currently, the viscous process is preferred. In this process, the cellulose is reacted with sodium hydroxide solution and then with carbon disulfide. This produces a yellow-orange-colored cellulose xanthogenate solution which is extruded through a spinneret. The cellulose is then regenerated using precipitation and washing baths. A variety of apparatuses have had to be developed for this for cleaning up exhaust air and wastewater.
As early as 1936 it was discovered that cellulose is soluble in oxides of tertiary amines (DE 713 486); however, this discovery was not pursued further until 30 years later. In the course of this, N-methylmorpholine N-oxide (NMMO) was identified as the most suitable solvent. The cellulose dissolves therein without being chemically modified. No breakdown of the cellulose chains takes place. Preparation of the corresponding spinning solutions has also been disclosed (DD 218 104; DD 298 789; U.S. Pat. Nos. 4,145,532; 4,196,282; 4,255,300). Yarns may be produced from the solutions by extrusion into a spinning bath (DE-A 44 09 609; U.S. Pat. No. 5 417 909). WO 95/07811 (=CA 2 149 218) also discloses a process for producing tubular cellulose films by the amine oxide process. A characteristic of this process is cooling the extruded film by cooling gas immediately below the annular gap of the extrusion die. According to EP-A 662 283, the extruded tubular film is cooled internally using liquid.
Recovery and purification of the NMMOs are described in DD 274 435. Since the cellulose is not chemically modified in the process, less equipment is required. In the amine oxide process, no gaseous or aqueous waste products are produced, so that there are no problems with respect to the exhaust air or the wastewater. It is therefore achieving increasing importance.
EP-A 0 686 712 describes the production of flexible cellulose fibers by the N-methylmorpholine N-oxide (NMMO) spinning process. In this process, a cellulose solution in aqueous NMMO is forced through a spinneret, conducted via an air section into an NMMO-containing aqueous precipitation bath and then washed, post-treated and dried.
According to WO 93/13670, a seamless tubular food casing is produced by extruding a solution of cellulose in NMMO/water using a special extrusion die. An air section is situated between extrusion die and precipitation bath. A characteristic of this process is a specially shaped hollow mandrel through which the precipitation liquid can also circulate in the interior of the tube. In the air section, the interior of the extruded tube is virtually completely filled by a hollow mandrel and precipitation liquid. The film is not stretched transversely in the course of this.
WO 95/35340 describes a process for producing blown cellulose films in which an underivatized cellulose dissolved in NMMO is used.
However, the amine oxide process also has disadvantages. The underivatized cellulose molecules are already preorientated in the NMMO solution and are substantially more tightly packed than is the case with chemically modified (xe2x80x9cderivatizedxe2x80x9d) molecules. On extrusion, the orientation in the longitudinal direction is still more pronounced. The yarns thus produced therefore exhibit a high strength in the longitudinal direction, but only low strength in the transverse direction. They have a strong tendency to split on being mechanically stressed in the wet state. Films or other shaped bodies, which must be able to be loaded in the longitudinal and transverse direction, may thus scarcely be produced by this method.
The object was therefore to modify the amine oxide process in such a manner that sufficiently load-bearing films or shaped bodies, in particular tubular food casings, can be produced. The process should succeed in this case with as few steps as possible, and should remain inexpensive and environmentally compatible.
The object can be achieved if the wet treatment is combined with a blow molding. The present invention thus relates to a seamless tubular cellulose-based film, which is obtainable by extruding a cellulose-, N-methyl-morpholine N-oxide- and water-containing spinning solution through an annular die and treating the tubular film in an N-methylmorpholine N-oxide-containing aqueous spinning bath, wherein the spinning solution comprises 0.2 to 50% by weight, based on the weight of the cellulose, of modifying compounds which increase the suppleness, strength, clipping stability and shear stability of the tubular casing.
The spinning solution preferably comprises 7 to 15% by weight, particularly preferably 9 to 12% by weight, cellulose, in each case based on the total weight of the spinning solution. The mean degree of polymerization of the cellulose in this case is preferably 300 to 700, particularly preferably 400 to 650. As solvent, the spinning solution preferably comprises 90.5 to 92.5% by weight NMMO and 9.5 to 7.5% by weight water. The parameters mentioned in this paragraph, together with the temperature, essentially determine the viscosity and fluid behavior of the spinning solution.
Processes for preparing the spinning solution are generally familiar to those skilled in the art. Customarily, cellulose is mashed in a 60% strength by weight aqueous NMMO solution at room temperature. The cellulose usually originates from wood or cotton. As the temperature increases, water is then distilled off in a heated stirred tank under reduced pressure until the residue consists of cellulose and NMMO monohydrate. This is the case at an NMMO content of 87.7% by weight, based on the total weight of NMMO and water. The ratio of NMMO to water may be readily determined by the refractive index. In the NMMO monohydrate, the cellulose dissolved completely at a temperature of 85 to 95xc2x0 C. with intensive stirring. The refractive index of the solution is 1.4910 to 1.4930. The water content has decreased to 7.5 to 9.5% by weight. The spinning solution is degassed, filtered and transferred to the spinning vessel.
The modifying compounds for improving the suppleness must be miscible with the cellulose/NMMO/water solution. The content of these compounds is preferably 0.5 to 20% by weight, particularly preferably 1 to 15% by weight, in each case based on the weight of the cellulose. The compounds may be mixed homogeneously with the spinning solution at a temperature of 85 to 105xc2x0 C., preferably 90 to 100xc2x0 C. Particularly suitable modifying compounds are starch, starch derivatives and cellulose derivatives (in particular esters or ethers of the starch or cellulose), as well as sugar esters, and in addition hydrophilic naturally occurring polymers (preferably alginic acid and alginates, chitosan and carrageenan). Suitable compounds are also hydrophilic synthetic polymers (preferably vinyl alcohol, vinyl acetates and acrylates) and polymers which simultaneously possess hydrophilic and hydrophobic properties (preferably esters from a sugar-such as sucrose-and fatty acids, the esters having an HLB of 1 to 15; HLB=hydrophilic-lipophilic balance). If appropriate, fatty acids and salts thereof, for example stearic acid or calcium stearate, waxes and paraffins are also suitable. Finally, polyvinylpyrrolidone, copolymers of vinylpyrrolidone and 2-(dimethylamino)ethylmethacrylate, copolymers of methyl vinyl ether and maleic anhydride or of methyl vinyl ether and maleic acid monoalkyl ester may also be used. The modifying compounds may also be crosslinkable, as is the case with polyethyleneimines. They also act as internal (primary) plasticizers. Impregnation with secondary plasticizers (such as glycerol) can frequently be even entirely omitted if the content of the modifying compounds in the food casings according to the invention is great enough (generally of the order of magnitude 8% by weight or more, based on the weight of the dry cellulose). Furthermore, they generally decrease the tendency of the cellulose to crystallize.
The spinning solution is extruded through the annular die preferably at a temperature of 85 to 105xc2x0 C., particularly preferably 90 to 95xc2x0 C. The annular gap is generally 0.1 to 2.0 mm wide, preferably 0.2 to 1.0 mm. The width here must be adapted to the warpage. xe2x80x9cWarpagexe2x80x9d is defined as the quotient of the velocity on leaving the annular gap (exit velocity) and the velocity at which the extruded tube is taken off (take-off velocity). The warpage is generally 3.0 to 0.10, preferably 2.0 to 0.2, particularly preferably 1 to 0.4. The exit velocity, depending on the construction of the plant, is 5 to 120 m/min, preferably 10 to 80 m/min. It is also determined by the caliber. On the extruded tube, advantageously, only a low tension is exerted in the longitudinal direction, which is essentially due to its own weight.
The xe2x80x9cair sectionxe2x80x9d, i.e. the section between annular gap and surface of the spinning bath in which the blow molding takes place, is preferably 1 to 50 cm, particularly preferably 2.5 to 20 cm. It is also dependent on the diameter (xe2x80x9ccaliberxe2x80x9d) of the tubular film after the blow molding. In contrast to the abovementioned WO 95/07811 and EP-A 662 283, no measures are required for additional cooling in the air section, and accordingly they are also not provided. The extruded tube cools only a small amount in the air section. Otherwise, transverse stretching would scarcely be possible. The blow molding is effected by compressed air or other gases which pass into the interior of the tube through orifices in the die body. Stretching in the transverse direction considerably increases the transverse strength of the tube. Depending on warpage, the diameter of the blow-molded tube is up to 100% greater or up to 50% smaller, preferably up to 80% greater or up to 20% smaller, than immediately after exiting the annular gap. Transverse stretching with a diameter simultaneously becoming smaller is obviously only possible if the warpage is less than 1. Preferably, the diameter of the blow-molded tube is 10 to 100% greater, particularly preferably 20 to 80% greater than immediately after exiting the annular gap.
If appropriate, the tube is conducted via a pipe, preferably a metal pipe. The diameter of this pipe can be selected between 30% greater and 30% smaller than that of the annular gap. Precipitation liquid and support air are fed via this pipe.
After entering the spinning bath, the diameter of the tube decreases. Through appropriate apparatuses in the die body, the spinning bath solution also passes into the interior of the cellulose tube. As a result, the tube solidifies more rapidly; at the same time, the insides are prevented from sticking together. The liquid level in the interior of the tube should not be significantly higher or lower than that of the surrounding spinning bath. The spinning bath itself is an aqueous solution which comprises 5 to 50% by weight, preferably 8 to 20% by weight, of NMMO. The temperature of the spinning bath is in the range from 0 to 50xc2x0 C., preferably 2 to 20xc2x0 C.
The depth of the spinning bath is determined by the caliber of the cellulose tube, its wall thickness and the desired residence time in the bath. Generally, the depth should be selected so that, on flattening the tube on the guide roll, the resulting edges are not damaged. In the case of a tube of caliber 20, which, immediately after leaving the annular gap, has a wall thickness of 0.5 mm and passes through the bath at a velocity of 20 m per minute, the spinning bath has a depth of about 3 m.
For further solidification, the laid-flat tube then passes through still more NMMO-containing precipitation vats. The first precipitation vat comprises approximately 10 to 20% by weight of NMMO. In the following precipitation vat, the NMMO content decreases. It has been found to be favorable to increase the temperature from one precipitation vat to the next, up to about 60 to 70xc2x0 C. in the last vat. The NMMO content in the tube is thus more greatly decreased.
This so-called xe2x80x9cprecipitation sectionxe2x80x9d is followed by water-filled wash vats, in which the last traces of NMMO are removed from the tube. The temperature of these baths is 15 to 70xc2x0 C., preferably 40 to 60xc2x0 C. Generally, a so-called plasticizer vat then follows. This comprises an aqueous solution of a plasticizer for cellulose. Suitable plasticizers are polyols and polyglycols, in particular glycerol. The aqueous solution comprises 5 to 30% by weight, preferably 6 to 15% by weight, of plasticizer. The temperature of the plasticizer solution is advantageously 20 to 80xc2x0 C., preferably 30 to 70xc2x0 C. The glycerol content of the casing is then about 15 to 30% by weight, preferably 18 to 23% by weight, in each case based on its total weight.
Thereafter, the tubes are conducted through a hot-air dryer in the inflated state. Expediently, drying is performed at decreasing temperature (from about 150xc2x0 C. at the inlet to about 80xc2x0 C. at the outlet of the dryer). An additional transverse orientation may be achieved, if appropriate, by appropriately increased internal pressure on drying. Otherwise, the tube is inflated on drying to the original caliber, in order to retain the degree of transverse orientation once achieved. During drying, the swelling value decreases to 130 to 180%, preferably 140 to 170%, depending on drying conditions and glycerol content. The tube is then wetted until the water content is 8 to 20% by weight, preferably 16 to 18% by weight, in each case based on the total weight of the tube. Then, using a pinch-roll pair, it can be laid flat and wound up.
Used aqueous NMMO solution may be purified by ion-exchange columns. The water can be taken off under reduced pressure until the NMMO concentration has reached 60% by weight. This NMMO solution can then be used again for preparing the spinning solution. The NMMO is thus virtually completely recovered.
Depending on caliber, the finished tubes, at a glycerol content of 20 to 22% by weight and a water content of 8 to 10% by weight, in each case based on the total weight of the tube, have a weight of 30 to 120 g/m2, preferably 35 and 80 g/m2. The weight per unit area generally increases with increasing caliber. The bursting pressure is likewise dependent on the caliber (small calibers have a higher bursting pressure). For a tube having a caliber of 16 mm, the bursting pressure is about 60 kPa, for a caliber of 30 mm about 40 kPa, at a caliber of 50 mm about 24 kPa and at a caliber of 140 mm about 15 kPa. The bursting pressure is measured in each case here in the wet state.
The tubular casings according to the invention can, furthermore, be provided on the inside and/or outside with an impregnation or coating, e.g. a liquid smoke impregnation or an xe2x80x9ceasy peelxe2x80x9d internal preparation. The same obviously applies to flat films.
An essential advantage of the flat or tubular films according to the invention is the uniform structure and thus uniform density which is achieved on precipitation. Films which are produced by the viscose process, in contrast, have a density gradient (higher density on the surface, lower in the interior).
The tubular films according to the invention are preferably used as sausage casings, in particular as xe2x80x9cpeelable casingxe2x80x9d in the production of frankfurters. In addition, they can also be used as membranes for various purposes, e.g. in hemodialysis. Finally, flat films can also be produced by cutting open the tubes.
If the cellulose tubes are used as sausage casings, the stuffing caliber can correspond to the annular gap diameter or up to 120% above it. Preferably, the stuffing caliber is 10 to 80% above the annular gap diameter.
The following examples serve for more detailed description of the invention. Percentages therein are percentages by weight, unless stated otherwise. Flat width, weight of the casing and thickness of the casing wall were determined under standard conditions (55% relative humidity; 23xc2x0 C.).